Automatic slack adjuster assembly for vehicle braking system

ABSTRACT

The invention pertains to the fluid powered brake systems of vehicles using brake drum and lined brake shoes internally expanded by cam. The invented device automatically adjusts the slack, which develops due to the wearing of brake liners and the drum. The device is depicted in FIG.  1  when the angular movement of the Lever Housing ( 1 ) exceeds the desired limit due to the wear, the internal device adjusts the excess clearance of slack automatically. The parts consist of Worm Wheel ( 3 ); an enmeshing Worm Shaft ( 4 ); a Paul ( 6 ); a Leaf Spring ( 1 ); a Ratchet cylinder ( 5 ); an Acuator Rod ( 7 ); a Guide ( 9 ); an End Connector ( 8 ); a Clevis ( 2 ); and Clevis Pins ( 2 A) and  2 (B). When brakes are applied the Actuator Rod ( 7 ) gets pulled out and brings the Paul ( 6 ) in engagement with Ratchet cylinder ( 5 ). On the return stroke the Paul ( 4 ) rotates the worm shaft ( 4 ) which in turn rotates the worm wheel ( 3 ) and the attached cam thereby removing the slack.

FIELD OF INVENTION

The present invention relates to a vehicle braking system and more particularly to an automatic slack adjusting assembly of novel construction and mode of operation, which actuates the cam for brake application in fluid operated brakes of medium and heavy vehicles. The novel aspect of the present invention adjusts the slack automatically, yet does not affect the provision of manual adjustment of the braking parts.

BACKGROUND AND PRIOR ART REFERENCES

The invention pertains to pneumatically powered brake system of medium and heavy vehicles. These vehicles invariably use internally expandable brake shoes lined with friction material and the braking effect is produced when these shoes make contact with a drum mounted on the driving or trailing wheels. The expansion of the brake shoes is done by S-profiled cams, which push the shoes out when the cam is rotated. The cam is rotated by lever arm at the end of which the force is applied by an air chamber, which gains its thrusting force, by compressed air. Hence, the name pneumatically operated brakes.

As the vehicles run, the brake is applied several times, which causes the wear of the friction lining of the brake shoes along with that of brake drum. During course of time, the clearance between the brake shoes and drum starts getting enlarged due to this wear. The enlarged clearance which is hereto called “Slack” demands more cam rotation, which in turn, needs larger stroke of the air chamber. The air chamber stroke is limited. A situation may arise when the full air chamber stroke would fail to expand the brake shoes enough for effective braking.

When the braking system of vehicle develops slack, it needs to be adjusted for effective braking. For carrying out this operation, the vehicle is to be grounded and adjustment to be made by trained mechanics or auto-work shops. This results in productive time loss. The slack adjuster is provided with Worm and Gear system in such a way that by rotating the Worm the s-cam also rotates and subsequently eliminates the slack i.e. the clearance developed between the brake shoes and the drum. The brake thus becomes effective again. This is normally termed as “brake adjustments”.

The vehicles need to be taken to a workshop for carrying out this brake adjustment which causes a lot of productive time of the vehicle. Any negligence in getting the adjustment done in time may also cause accidents due to non-effective braking. The brake adjustments are required to be done several times during the life of the brake linings, resulting in loss of several productive days. Besides the effectiveness of braking may also get impaired if timely adjustments are not carried out.

To circumvent the problem of manual adjustment of brakes periodically, few manufacturers of Slack Brake Adjuster have incorporated a system to automatically adjust the slack or the clearance between the brake shoes and the drum. These systems use several parts like small gears, racks, coil springs, clutches and retainers, etc. The applicants of this patent have also designed an Automatic Slack Adjuster but using very few parts and no clutch system. The initial mounting and setting the assembly on the brake system of vehicle has also been simplified and thus does away the necessity of the lifting pauls or work against unidirectional clutches.

In patent number GB1146965, the patentee describes sensing from external anchoring at chassis. In the prior art application use of clutches, gear trains secondary primary worms results in slack adjustment. The construction features of the apparatus used for slack adjustment as defined in the prior art are different from the present invention.

In patent number U.S. Pat. No. 5,350,043, an automatic slack adjuster operating to effect reduction of slack present in a vehicle brake system upon brake release is defined. The slack adjuster incorporates a member cooperating with a slack adjusting rotor to determine a reference position for the rotor and to return the rotor to such reference position at the completion of each brake operational cycle. Slack adjuster 10 includes an elongated housing 20 having a bore opening adjacent one end for receiving pivot pin 16 and a bore opening adjacent an opposite end for rotatably supporting a worm gear 22, which is suitably keyed to cam shaft 18, as by a spline connection 24, for rotation about a first axis 26. Slack adjuster 10 is also connected to clevis 14 via a link 28, which is slidably supported by housing 20 and has a protruding end pivotally connected to the clevis by a pivot pin 30. Such a slack adjustment is incorporated with sensing by Clevis, rotation of worm by heavy spring loaded clutch and rotor, internal worm mounted cam, pushes and springs loaded stopper. The system as described in the prior art is extremely complex and makes the system cumbersome. The complexity of the system makes the prior art system extremely difficult to implement with reduced efficiency of the system as a whole.

In publication number WO03083322, an automatic brake adjuster for adjusting the slack between the brake lining and brake drum of a vehicular braking system is defined, which comprises of a housing (1) accommodating an internally splined worm wheel (3) for receiving the S-cam shaft of the system; a worm shaft (4) positioned perpendicularly to the axis of the worm wheel (3) and meshing therewith; a control arrangement (12, 13, 14, 15 and 17) for controlling the movement of a clutch worm wheel (6), depending upon the angular movement of the adjuster, clutch worm wheel (6) being mounted on the worm shaft (4) and forming a clutch by serration (6′) of the clutch worm wheel engaging with the serration (4″) of the worm shaft, under the influence of a heavy compression spring (9), the control gear (12) located in the worm wheel bore of the body (1) being permanently fastened to the control arm (13) having a fixing arrangement (13′) for rigidly fixing to the chassis of the vehicle, to establish a reference; a pinion assembly comprising a ratchet (16′) and pawl (26) mechanism between the pinion (16) and the control worm screw (19), Guide (20), screw (21), spindle (23) and spring (24) and positioned parallel to the axis to control gear wheel (12) and perpendicular to the axis of worm shaft (4), the pinion (16) meshing with the control gear wheel (12) and being guided in the body (1), whereby the ratchet (16′) and pawl (26) mechanism provided between pinion (16) and control worm screw (19) functions as one-way lock and allows free rotation of pinion (16) during counter clockwise rotation under resistance from the pawl spring (27) to prevent over-riding while clutch worm gear (6) is in disengaged condition the ratchet (16′) and pawl (26) mechanism also permitting required axial movement of the control worm screw (19) to control the desired slack, the excess lining clearance being thus sensed to reference point and adjusted by said ratchet (16′) and pawl (26) mechanism to maintain optimum brake lining clearance. Hence, the applicant of the prior art discloses sensing from external anchoring at chassis Worm mounted spring to loaded clutch connected to the secondary worm and gear to provide rotation to the primary worm, which is not the case with the present invention.

Such prior devices had certain disadvantages as mentioned below:

-   -   Higher cost     -   Too bulky and needs modifications for fitment or need external         anchoring bracket with a chassis.     -   Use too many parts and miniature clutch systems which tend the         wear out and malfunction.     -   The certain devices could not be sealed properly against ingress         of water/water mists and dust thereby rendering the parts to         wear and seize.     -   Some devices are not interchangeable for left and right side         application and the direction of thrust.

OBJECTS OF THE INVENTION

Accordingly, the primary object of the invention is to provide a novel automatic slack adjuster in vehicle braking system.

Another object of the present invention is to provide a unidirectional worm-rotating device so that the worm once rotated does not revert to its original position.

Another object of the present invention is to provide a constant incremental adjustment through 360° of the cam rotation though normally the cam does not rotate more than 120° during the lifetime of the brake living.

Another object of the invention is to provide nearly constant free movement of the lever housing to a predetermined angular range within which the automatic mechanism remains dormant thereby avoiding over-adjustment.

Yet, another object of the present invention is to provide optimum clearance between brake lining and drum surface.

Another object of the present invention is the interchangeability for right hand and left-hand application.

Another object of the present invention is easy adaptability, initial mounting and adjustment in the existing brake system without alteration.

Another object of the present invention is to provide a construction, which could be sealed against the ingress of dust and moisture, and to retain the lubricants inside the assembly.

SUMMARY OF THE INVENTION

The present invention leads to rotation of the worm, which in turn rotates the engaging gear coupled to the s-cam shaft. The rotation of the worm is to take place only when the clearance between the brake shoes and drum increases to a predetermined extent. The worm-rotating device also needs to be unidirectional so that the worm once rotated does not revert to its original position. In order to achieve this basic requirement the worm has been designed with an integrated ratchet cylinder, which can be rotated only in one direction by a paul. The Linear movement of the paul is given by an actuator rod, linked by a clevis to the push rod of the air chamber which actuates the brake.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: illustrates the slack adjuster of GB1146965 having cluthes, gear trains, secondary primary worms.

FIG. 2: illustrates the slack adjuster of U.S. Pat. No. 5,350,043 having clevis, spring loaded with clutch and rotor and internal worm mounted cam.

FIG. 3: illustrates the slack adjuster of WO03083322 having external anchoring, secondary worm and gear assembly.

FIG. 4: illustrates brake adjuster assembly in initial position

FIG. 5: illustrates front view and top view of brake adjuster assembly in initial position

FIG. 6: illustrates front view and top view of brake adjuster assembly in brake applied position

DETAILED DESCRIPTION OF THE INVENTION

Accordingly, the present invention relates to slack adjuster, which automatically adjusts the slack between the brake drum and the brake lining riveted to the brake shoes. The said brake shoes expand with the rotation of the S-profiled cam.

In FIG. 6, the said automatic slack adjuster assembly for vehicle braking system comprises of

-   -   (a) a lever housing (1) accommodating a worm-wheel means having         splined bore to receive camshaft, said worm-wheel means         comprising a worm in functional relationship with a wheel (3);     -   (b) a worm shaft (4) having ratchet cylinder (5) portion,         enmeshed in said worm at right angle to the axis of the wheel;     -   (c) a paul (6) positioned over the ratchet cylinder (5) portion         of the worm shaft (4) inside a machined guideway in the housing         (1);     -   (d) a leaf spring (10) located on one end of the machined         guideway pressing down the said paul;     -   (e) an actuator rod (7) linked to the said paul (6) runs inside         guide (9), opposite to the end of the leaf spring guideway,         provides linear motion to the paul (6);     -   acuator rod (7) means connected to an end connector, said end         connector being connected to a clevis (2) by means of a clevis         pins (2B),     -   (f) the clevis (2) is attached externally to the said Lever         Housing (1) by clevis pin (2A), and     -   (g) a force exerting means, connected to the said clevis pin         (2A) for providing necessary force.

In yet another embodiment of the present invention, the wheel in the worm-wheel means is a gearwheel.

In another embodiment of the present invention, said assembly is sealed using a sealing assembly, said sealing assembly comprising cover (14); gasket (12); O rings (13); worm shaft seal (11); actuator rod O-Ring (15) and bellows (not shown).

In another embodiment of the present invention, said sealing assembly avoids ingress of dust and moisture and retains lubricants inside the assembly.

In another embodiment of the present invention, paul is a special engineered material single tooth bar.

In another embodiment of the present invention, the unidirectional paul and ratchet drive is utilized to achieve the adjustment of slack automatically in cam operated vehicle brakes.

In another embodiment of the present invention, off-setting of paul tooth away from the center line of the ratchet cylinder determines the amount of angular rotation of the lever housing during which automatic adjustment is not desired.

In another embodiment of the present invention, wherein the tooth position in the paul is off set so that the paul does not engages with the ratchet cylinder for a pre-determined angular movement of the said lever housing.

In another embodiment of the present invention, wherein the predetermined angular movement is preferably in the range of 12° to 16° In another embodiment of the present invention, the paul is a special engineered toothed plate.

In FIG. 4, the rotation of the camshaft through angle A when the brake adjuster assembly is pushed by the air chamber through push rod and clevis is shown. The actuator rod in brake applied position gets pulled out and in turn places the Paul (6) in engaging position with Ratchet Cylinder (5) teeth as shown in FIG. 6. When the brake is released and the brake adjuster reverts into the initial position by the air chamber, the Paul (6) rotates the worm shaft, which in turn rotates Worm wheel (3) thereby adjusting the slack. The offsetting of Paul (6) tooth ensures that its engagement with Ratchet cylinder (5) only after a certain angular movement of the brake adjuster assembly. When slack is not developed, the brake adjuster angular movement is less which does not allow the Paul (6) to ride over the Ratchet cylinder (5) and therefore the Worm Shaft (4) does not rotate and thereby avoids the adjustment to take place. When the angle is increased, which happens when the slack is developed, the adjustment takes place.

Thus, the mechanism maintains the desired clearance between the brake lining and the drum.

In yet another embodiment of the present invention, the unidirectional Paul (6) and Ratchet (5) drive is utilized to achieve the adjustment to Slack automatically in cam operated vehicle brakes.

In yet another embodiment of the present invention, off-setting of Paul (6) tooth away from the center line of the Ratchet Cylinder determines the amount of angular rotation of the Lever Housing (1) during which automatic adjustment is not desired.

In yet another embodiment of the present invention, the application of unidirectional Paul and ratchet drive to achieve the adjustment of slack automatically in pneumatically operated vehicle brakes.

The Slack Adjuster assembly is mounted over the cam shaft splines. It is linked to the push rod of the air chamber by means of a clevis having two holes, one big and other small as given in FIG. 4. When compressed air is fed to the air chamber, the linear outward motion of the push rod gives angular movement to the slack adjuster assembly which in turn rotates the cam shaft as give in FIG. 5, thereby expanding the brake shoes for brake application. It is evident from the figures that the distance between the small clevis hole to the worm axis gets extended. This extension pulls out the actuator rod. The FIG. 4 shows the superimposed view of the slack adjuster assembly in “initial” or home position and in “brake applied position. The extended distance or the amount by which the actuator rod gets pulled out is in direct proportion to the angle “A” of the cam shaft rotation. Lesser the angle lesser is the pull out amount of the actuator rod.

The actual construction details of the slack adjuster assembly are shown in FIG. 5 and FIG. 6. FIG. 5 shows the initial of the home position and FIG. 6 shows when the actuator rod gets pulled out to the maximum. In this position the paul tooth gets engaged with the ratchet cylinder on the worm and when the assembly retracts back to the “home” position the paul pushes the ratchet thereby making the worm to rotate. The worm then rotates the gear and the gear, coupled to the camshaft, rotates the cam.

Initially, when the brake linings are new, the clearance between the brake shoes and drum is the minimum and requires lesser angular movement of the slack adjuster for effective brake application. The tooth position in the paul is sufficiently offset so that for movement up to about 12/14 degrees does not make the paul tooth engage with the ratchet. In this range, the paul body just slides over ratchet teeth. The leaf spring allows the paul to lift-up for engagement but also exerts sufficient force to avoid its disengagement when operating.

As the wear on lining and drum takes place the clearance between the brake shoes and drum increase causing the arm movement exceeding 14°, which also includes the angular shift caused by the flexing the brake systems parts such as cam shaft, shoes, bearings clearances etc.

As and when this happens the paul comes into function and reduces the clearance by rotating the cam shaft till the arm movement falls below 14°. The automatic adjustments therefore remain dormant till the arm movement again exceeds 14°. With the automatic adjustment in function the arm movements are limited around 14°/16° only and hence automatically control the air chamber stroke. This results in reduced air consumption and limited flexing of the air chamber diaphragm.

Other advantages include

-   -   Gain in productive days of vehicles because manual adjustments         are not needed.     -   Elimination of brake adjustment costs.     -   Uniformity of brake performance because the clearance are         maintained optimally.     -   Greater reliability. 

1. An automatic slack adjuster assembly for vehicle braking system, said brake adjuster assembly comprising: (a) a lever housing (1) accommodating a worm-wheel means having splined bore to receive camshaft, said worm-wheel means comprising a worm in functional relationship with a wheel (3); (b) a worm shaft (4) having ratchet cylinder (5) portion, enmeshed in said worm at right angle to the axis of the wheel; (c) a paul (6) positioned over the ratchet cylinder (5) portion of the worm shaft (4) inside a machined guideway in the housing (1); (d) a leaf spring (10) located on one end of the machined guideway pressing down the said paul; (e) an actuator rod (7) linked to the said paul (6) runs inside guide (9), opposite to the end of the leaf spring guideway, provides linear motion to the paul (6); an actuator rod means connecting to an end connector, said end connector being connected to a clevis (2) by means of a clevis pins (2B), (f) the clevis (2) is attached externally to the said Lever Housing (1) by clevis pin (2A), and (g) a force exerting means, connected to the said clevis for providing necessary force.
 2. The automatic slack adjuster assembly as claimed in claim 1, wherein said assembly is sealed using a sealing assembly, said sealing assembly comprising cover, gasket, O-rings, worm shaft seal, actuator O-ring and bellows for avoiding ingress of dust and moisture and to retain the lubricants inside the assembly.
 3. The automatic slack adjuster assembly as claimed in claim 1, wherein the wheel in the worm-wheel means is a gear wheel.
 4. The automatic slack adjuster as claimed in claim 1, wherein the force exerting means is a fluid chamber having a push rod.
 5. The automatic slack adjuster assembly as claimed in claim 1, wherein the uni-directonal paul and ratchet drive is utilized to achieve the adjustment of slack automatically in cam operated vehicle brakes.
 6. The automatic slack adjuster assembly as claimed in claim 1, wherein the off-setting of paul tooth away from the centre line of the ratchet cylinder determines the amount of angular rotation of the lever housing during which automatic adjustment is not desired.
 7. The automatic slack adjuster assembly as claimed in claim 5, wherein the tooth position in the paul is off set so that the paul does not engages with the ratchet cylinder for a predetermined angular movement of the said lever housing.
 8. The automatic slack adjuster assembly as claimed in claim 6, wherein the predetermined angular movement is in the range of 12° to 16°.
 9. The automatic slack adjuster assembly as claimed in claim 6, wherein the predetermined angular movement is preferably 14°. 